Redesign improves energy efficiency of supermarket cold food cases

Engineers used physics to improve the designs of open-air refrigerated display …

We’ve all been there, standing in the supermarket's cold foods aisle, trying to decipher the fruit juice cocktails (cranberry-orange-pineapple? banana-grape-apple?) and catching a chill during the process. The open-air refrigerated display cases are awfully convenient for reading the (complicated) labels, since they've got no glass to fog up. But the air that is making you cold isn’t keeping the food cold, so this represents a loss when energy is required to cool replacement (warmer) air. Up to three-quarters of the energy for cooling in these cases is used to make up for warm air mixing into the cold.

Since open-front formats make up about 60 percent of all refrigerated display cases, this represents a significant amount of energy. To try and reduce some of this wasted energy, a team of engineers at the University of Washington and Kettering University led by Mazyar Amin used physics to improve the designs of open-front refrigerator cases (it was his thesis project). In a recent paper in Applied Thermal Engineering, they show that energy usage can be reduced by 10 to 15 percent with simple modifications—and if these changes are made nationwide, hundreds of millions of dollars in energy costs could be saved.

Refrigerated display cases in supermarkets are ubiquitous, practically invisible pieces of technology—but use about 36 percent of a typical supermarket’s energy. On average, US grocery stores use about 194 kWh per m2 on refrigeration. This works out to 828,000 kWh in a year (and $87,600 based on an average cost of energy of $0.1058 per kWh) for a store of the median national size. With over 30,000 larger supermarkets in the US, that’s almost $3 billion on refrigeration alone. Obviously, even a small reduction in energy usage means a huge savings nationwide (and possibly thousands per store).

Open-air refrigerator cases actually use something called an air curtain to help keep the cold air in and the warm air out. This is a jet of cold air blown down across the opening as a sort of shield—these significantly decrease the amount of warm air mixing in compared to a completely open case, but they aren’t perfect. In fact, the air curtain can actually capture some warm air (entrainment), which you want to minimize. The engineers showed that, by changing a couple of controlling factors, you can do just that.

They built an air curtain simulator to test out various combinations of design parameters, including the angle of the air jet (throw angle), the angle of the return air grille relative to the jet (offset angle), and the ratio of the case opening to the jet opening width. They also tweaked the the Reynolds number (the ratio of momentum to viscous forces, equal to the jet mass flow rate divided by the jet opening width and viscosity of air), which determines when a flow becomes turbulent.

These are actually just the primary variables controlling the infiltration of warm air—there are a number of secondary variables that have less of an impact, including the temperature difference between the jet and ambient air, the relative humidity of the ambient air, and the amount of food on the shelves. The researchers plan on considering these factors in a later study.

After measuring the infiltration of warm air for 576 different combinations of variables, they found a couple general trends, although the relationship between the four primary factors is complicated.

In general, a lower jet Reynolds number leads to a lower entrainment of warm air. This makes sense, because lower turbulence would cause less turbulent mixing, and therefore capture less warm air. The Reynolds number can (practically) be decreased by lowering the air jet mass flow rate or by increasing the width of the jet nozzle.

They also found that, in general, a lower offset angle and lower opening height to jet width ratio led to lower entrainment. These correspond to a smaller distance for the air jet to travel—meaning less distance over which to capture warm air.

After performing these measurements, the engineers retrofitted an existing refrigerated case with relatively low-cost changes and showed a 10 percent reduction in warm air infiltration. In other cases, the potential savings reach 15 percent, and this reduction corresponds roughly to the reduction in energy usage.

Using the team’s results, manufacturers could optimize their designs to improve the air curtains and minimize the energy usage of refrigerated display cases, leading to a potential savings of $100 million or more in the US alone (if all cases nationwide were improved). It’s not clear exactly how feasible this is, but if even minor changes using the principles outlined in this paper improve the energy usage a few percent, surely that’s worth the effort in the long run?

Kyle Niemeyer / Kyle is a science writer for Ars Technica. He is a postdoctoral scholar at Oregon State University and has a Ph.D. in mechanical engineering from Case Western Reserve University. Kyle's research focuses on combustion modeling.